As the development of nanoscale mechanical, electrical, chemical and biological devices and systems increases, new processes and materials are needed to fabricate nanoscale devices and components. Making electrical contacts to conductive lines has become a significant challenge as the dimensions of semiconductor features shrink to sizes that are not easily accessible by conventional lithography. Optical lithographic processing methods have difficulty fabricating structures and features at the sub-60 nanometer level. The use of self assembling diblock copolymers presents another route to patterning at nanoscale dimensions. Diblock copolymer films spontaneously assemble into periodic structures by microphase separation of the constituent polymer blocks after annealing, for example, by thermal annealing above the glass transition temperature of the polymer or by solvent annealing, forming ordered domains at nanometer-scale dimensions.
The film morphology, including the size and shape of the microphase-separated domains, can be controlled by the molecular weight and volume fraction of the AB blocks of a diblock copolymer to produce lamellar, cylindrical, or spherical morphologies, among others. For example, for volume fractions at ratios greater than about 80:20 of the two blocks (AB) of a diblock polymer, a block copolymer film will microphase separate and self-assemble into periodic spherical domains with spheres of polymer B surrounded by a matrix of polymer A. For ratios of the two blocks between about 60:40 and 80:20, the diblock copolymer assembles into a periodic hexagonal close-packed or honeycomb array of cylinders of polymer B within a matrix of polymer A. For ratios between about 50:50 and 60:40, lamellar domains or alternating stripes of the blocks are formed. Domain size typically ranges from 5-50 nm.
Attempts have been made to control orientation and long range ordering of self-assembling block copolymer materials. Salts such as sodium and potassium chloride (NaCl, KCl) have been shown to improve long range ordering of block copolymer material on substrates. However, the sodium (Na) and potassium (K) are highly mobile, which can result in contamination of other device structures during processing. Other researches have added organic surfactants to diblock copolymers to improve long range ordering during self-assembly. However, during the high temperature/vacuum anneal of the block copolymer material, the organic surfactant evaporates from the film before the self-assembly process is completed, limiting the annealing conditions that can be used.
It would be useful to provide methods of fabricating films of ordered nanostructures that overcome these problems and provide enhanced long range ordering of the self-assembling polymer domains.